Many underwater systems comprise distributed nodes working in concert. There is often an operational need to reference all the distributed nodes to a common system clock. For example, seismic survey systems employ remotely deployed sensors distributed over several hundred meters range. Each sensor independently takes acoustic measurements. Data processing aims to identify features below the seabed based on the relative time of arrival of acoustic signals at each sensor. The quality of the survey data is therefore dependant on how accurately the system can calculate relative timings of detected acoustic events.
Currently a common sinusoidal clock reference signal is provided through direct cable connections between the sensors or synchronisation is via acoustic transmission. However, cabled systems can be very problematic in the underwater environment. They require connection into the sensor housing, which can lead to water ingress. Also, above water pre-assembly of the connections leads to an unwieldy cabled structure, which is difficult to deploy, whilst connection below the water requires wet mate radio frequency connectors that are extremely unreliable.
In an alternative arrangement, an acoustic wireless system could be used. This removes the interconnect issues. However, because sound travels through water at around 1500 m/s the propagation time between sensor locations has to be taken into account. This requires accurate knowledge of the relative position of each sensor. Accurate relative positioning introduces a secondary technological problem in the underwater environment that is difficult to resolve and limits relative timing accuracy. For example, where sensors are spaced 100 m apart there could easily be +/−1 m error in known relative position. In an acoustic system, this would result in a timing error in the order of +/−667 microseconds.